The fact that an atom will emit a photon of radiation when one of its electrons drops to a lower energy state has enabled the laser to be employed in a number of military, industrial, and medical applications. The term "laser" is an acronym for light amplification by stimulated emission of radiation. In its simplest form, a laser consists of a rod of transparent crystal or a tube filled with gas or liquid. A mirror is placed at one end and a half-silvered mirror at the other end. The laser is then "pumped" by adding energy, e.g., by shining another light source into it, by adding electrical energy, or by stimulating a chemical reaction. This process raises electrons in the laser to higher energy states.
During the pumping process, some of the electrons will spontaneously fall back to a lower energy state, emitting photons. The photons that travel toward the sides of the laser are quickly lost, but those traveling along the length of the rod or tube are reflected back by the mirrors. This activity generally occurs in the area commonly referred to as the laser gain region. When these photons strike other excited atoms, they stimulate those atoms to release photons of the exact same energy level (or wavelength), which travel in the same direction as the stimulating photons. The result is an intense, highly focused beam of light escaping through the half-silvered end of the laser. This light beam is generally referred to as a beam of high energy coherent radiation, or more commonly, a laser beam.
Because the photon wavelength is determined by the characteristics of the atoms in the lasing material, laser light is of a single wavelength. Because laser light travels in a tight beam, it can carry a great deal of energy over a great distance without significant loss. With respect to recent developments in laser technology, there has been increased interest in chemical laser systems, especially COIL systems.
The COIL system was initially developed for military applications;
however, recent attention has turned to potential industrial uses of COIL systems, such as metal cutting applications.
In a COIL system, chlorine gas reacts with a solution of basic hydrogen peroxide (i.e., usually KOH or NaOH mixed with hydrogen peroxide) to produce a gaseous flow of excited oxygen, also referred to as singlet delta oxygen or singlet molecular oxygen (designated as O.sub.2 *, O.sub.2 (.sup.1.DELTA.), as well as by other symbols). This gaseous flow of singlet delta oxygen collides (typically at speeds approaching or even exceeding the speed of sound) with iodine gas molecules (i.e., I.sub.2), dissociating them and exciting the resulting iodine atoms (i.e., I), which lase at 1.315 .mu.m. The major laser byproducts are various salts (e.g., NaCl or KCl) and heat. The small amounts of iodine can be scrubbed from the output.
The intended operation of a conventional COIL system can be summarized as follows. The initial step is to generate the singlet delta oxygen. This is accomplished by providing a source of basic hydrogen peroxide, typically in liquid form, and a source of molecular chlorine, typically in gaseous form. These two materials are then charged or injected into a singlet delta oxygen generator through appropriate manifold/conduit assemblies, respectively. The resulting exothermic reaction between the basic hydrogen peroxide liquid and the molecular chlorine gas produces the gaseous singlet delta oxygen, as well as several by-products, such as salt and heat. The heat can be removed by appropriate devices such as a heat exchanger, and the salt can be removed by appropriate devices such as a scrubber.
Once the gaseous singlet delta oxygen is generated, it is then charged or injected in flow form into a mixing nozzle at the appropriate time. The mixing nozzle has a throat portion which generally divides the mixing nozzle into a subsonic zone and a supersonic zone; that is, the flow of gaseous singlet delta oxygen is subsonic in one portion of the mixing nozzle and supersonic at the other portion of the mixing nozzle. The mixing of reactants is typically done in the subsonic zone.
A molecular iodine generator is in communication with the mixing nozzle by an appropriate manifold/conduit assembly. At the appropriate time, gaseous molecular iodine is then charged or injected into the mixing nozzle in such a manner so as to let it "pool" before completely mixing with the singlet delta oxygen gas flowing from the singlet delta oxygen generator. The slight delay in mixing due to this "pooling" permits the singlet delta oxygen to dissociate only some of the molecular iodine on the edge of the "pool" and thus initiate the chain reaction dissociation by the product atomic iodine.
The primary reactions taking place in connection with the conventional COIL system are as follows:
(1) I.sub.2 +O.sub.2*.fwdarw.I.sub.2 *+O.sub.2. That is, a mole of molecular iodine reacts with a mole of singlet delta oxygen to produce a mole of excited molecular iodine and a mole of molecular oxygen. PA0 (2) I.sub.2 *+O.sub.2 *.fwdarw.2I+O.sub.2. That is, a mole of excited molecular iodine reacts with a mole of singlet delta oxygen to produce two moles of atomic iodine and a mole of molecular oxygen. PA0 (3) I+O.sub.2 *.fwdarw.I*+O.sub.2. That is, a mole of atomic iodine reacts with a mole of singlet delta oxygen to produce a mole of excited atomic iodine and a mole of molecular oxygen. PA0 (4) I*.fwdarw.I+hv. That is, a mole of excited atomic iodine releases a photon (hv), thus producing a mole of atomic iodine.
The singlet delta oxygen gas flow initially contacts the gaseous molecular iodine "pool" at subsonic speed; however, the singlet delta oxygen gas flow is quickly brought up to near supersonic or even supersonic speed (via appropriate devices such as a venturi) and is expelled out through the mixing nozzle into the area known as the laser gain region. It is in this area where the excited atomic iodine releases its photon. The released photon is then reflected many times between a set of mirrors, the first mirror being fully reflective, the second mirror being partially reflective. The reflected photons eventually form a laser beam, which is transmitted through the partially reflective mirror at a wavelength of 1.315 .mu.m. Any remaining chemical species are removed from the laser gain region by a combination of exhaust assemblies and scrubber assemblies in order to avoid contamination of the laser's mirrors.
At times, the COIL system experiences unpredictable and unexplainable anomalous freezing of the potassium basic hydrogen peroxide (K-BHP) even though the system was operating at temperatures 10.degree. C. above the known freezing point of the BHP. The formation of solids in the BHP plugs the singlet oxygen generator and causes the laser to cease operating. That fact that the system operates without a problem on some occasions, and at the same conditions it unexpectedly freezes on other occasions has defied explanation since the system is operating well above the known freezing point of BHP. This problem has occurred at numerous COIL system facilities and has been an ongoing problem for the COIL system for several years.
Accordingly, there has been increased interest involving the part of the process that involves BHP. Generally, the BHP is prepared by reacting aqueous solutions of KOH and H.sub.2 O.sub.2 to form aqueous KOOH. The accepted phase diagram for the KOH, H.sub.2 O.sub.2, H.sub.2 O system was determined by Dobrynina, et al., Bulletin of the Academy of Sciences, USSR, Division of Chemical Sciences, Volume 34, Page 2451 (1968). The phase diagram is shown in FIG. 1 after re-plotting their data on a KOH, H.sub.2 O.sub.2, and H.sub.2 O axis system.
Referring to FIG. 1, a phase diagram for K-BHP, expressed in weight percent (wt. %), is shown. The triangle drawn with solid lines represents the domain of 0% to 100% concentration for the KOH, H.sub.2 O.sub.2, H.sub.2 O system. The dashed lines correspond to negative concentrations of H.sub.2 O. Any point in the figure represents a mixture of the three components. The sum of the three constituents at any point adds up to 100%. The open circles show the composition of various compounds. The curved lines (referred to as isotherms) show the composition where the liquid phase is in equilibrium with the solid phase at a given temperature. The isotherm temperatures are labeled around the edge of FIG. 1. If the temperature is cooled below the equilibrium or saturation temperature, a solid compound will crystallize out of the solution and the composition of the remaining liquid will change accordingly. The reason FIG. 1 has negative compositions is that it has been re-plotted on a different axis system than Dobrynina, et al., who used an axis system based on K.sub.2 O, 1/2O.sub.2, and H.sub.2 O. Some of the compounds shown in Dobrynina, et al. are located in the negative area of FIG. 1. However, the KOH, H.sub.2 O.sub.2, H.sub.2 O axis system shown in FIG. 1 is more convenient for preparing BHP because these are the actual reagents used to prepare BHP, and the relative amounts of the three components can be read directly off of FIG. 1, without having to perform any calculations.
As previously noted, BHP is typically prepared by mixing aqueous KOH and aqueous H.sub.2 O.sub.2. KOH is commercially produced as a 50% solution, and is generally available as a 50% or 45% solution. The 45% KOH is most commonly used because 50% KOH freezes in cold weather. The strongest commercially available hydrogen peroxide is 70% H.sub.2 O.sub.2. BHP is typically made from these commercially available reagents. Their composition is shown in FIG. 2.
Referring to FIG. 2, dashed lines are drawn between 45% KOH and 70% H.sub.2 O.sub.2 and between 50% KOH and 70% H.sub.2 O.sub.2. The compositions that can be prepared by mixing the KOH solution with the H.sub.2 O.sub.2 solution must lie on the corresponding mix line.
Several factors determine the desired operating conditions. At ambient temperatures, BHP decomposes readily liberating a large amount of heat. Therefore, the BHP temperature needs to be below ambient temperature, but above the BHP saturation or freezing temperature. Because water vapor quenches the excited iodine, it is desirable to minimize the partial pressure of the water vapor in equilibrium with the BHP. This means that the BHP temperature should be kept as low as practical without freezing. However, if the temperature is too low or too close to the freezing point, the viscosity will be high and the kinetics will be slow. If the mixture is basic (i.e., has excess KOH), the production of singlet oxygen decreases. Consequently, the mole ratio of H.sub.2 O.sub.2 to KOH should be one or greater (stoichiometric or excess H.sub.2 O.sub.2).
The two black dots on the two mix lines in FIG. 2 represent typical operating conditions. The composition of the two black dots is as follows. On the 45% KOH mix line, the black dot is at 28 wt. % KOH, 26 wt. % H.sub.2 O.sub.2, and 46 wt. % H.sub.2 O which corresponds to 7 M KOH and 10.5 M H.sub.2 O.sub.2 or 7 M KOOH and 3.5 M excess H.sub.2 O.sub.2 (in the subsequent description, the abbreviated term "7M/10.5M BHP" will be used to express the composition). On the 50% KOH mix line, the black dot is at 29 wt. % KOH, 30 wt. % H.sub.2 O.sub.2, and 41 wt. % H.sub.2 O which corresponds to 7.2 M KOH and 12.3 M H.sub.2 O.sub.2 or 7.2 M KOOH and 5.1 M excess H.sub.2 O.sub.2.
Operating in the vicinity of the two black dots satisfies the requirements mentioned above. They both have excess H.sub.2 O.sub.2. According to phase diagram in FIG. 2, the two black dots have BHP freezing points of about -31.degree. C. to -32.degree. C. When operating the COIL system at a BHP temperature of -20.degree. C., they both have low water vapor pressure, and both have a very large safety margin against freezing because the operating temperature (-20.degree. C.) is more than 10.degree. C. above the BHP freezing point (i.e., -31 to -32.degree. C. according to FIG. 2).
COIL lasers have been operated successfully at these conditions for many years. However, they have also experienced unpredictable and unexplainable anomalous freezing incidents at the same conditions, even though the temperature is IOC above the BHP freezing point.
Suspecting unknown impurities as a possible cause, operators of COIL systems have cleaned their systems and replaced their reagents. However, this has not solved the problem.
Another approach to overcoming this problem is based on the order of mixing. Because BHP is less stable at high pH, it is normally prepared by adding the KOH to the H.sub.2 O.sub.2. It has been suggested that when BHP is prepared in this manner, the composition passes through a maximum in the freezing point en route to the final composition (as shown in FIG. 2). This approach hypothesizes that nucleation seeds can form in the vicinity of this maximum, and because of the high viscosity, once these seeds have formed they can survive even though the composition has proceeded away from this maximum. The approach further hypothesizes that the seeds will not form and anomalous freezing incidents can be prevented: A) by keeping the mix temperature well above the maximum, say above -13.degree. C.; or B) by adding the H.sub.2 O.sub.2 to the KOH the system never passes over the maximum. Working with 7M/10.5M BHP, proponents of this approach have found that they have not had a freezing incident since the adoption of this approach. However, by the very unpredictable nature of the "anomalous freezing" phenomenon, this conclusion is premature. Furthermore, tests indicate that BHP prepared in this way is metastable and will crystallize at -20.degree. C. when nucleation seeds form or are introduced.
Typically, only specific chemical compounds are effective as nucleation seeds. After an unexplained freezing incident, researchers analyzed the slush that formed. The result was inside the solid triangle of FIG. 1, specifically near the middle of the bottom of the solid triangle. Choosing one of the solid compounds listed on FIG. 1, they suggested that the solid might be K.sub.2 O.sub.2.2H.sub.2 O.sub.2. However, tests have indicated that the actual compound is not listed on FIG. 1.
Based on these results, the inventor of the present invention attempted to prepare this solid. A mixture of 43.8% H.sub.2 O.sub.2, 43.0% KOH, and 13.2% H.sub.2 O was prepared at -20.degree. C. Initially it was a thin slurry, but on standing over a weekend a large mass of crystals formed. When BHP at -20.degree. C. was seeded with this material, crystals precipitated.
Referring to FIG. 3A, a flask containing BHP after 2.5 hours at -200C but before seeding, is shown. It is a clear liquid with no solids. Six minutes after the seed was introduced, the temperature of the BHP began to rise noticeably.
Referring to FIG. 3B, the same flask containing the BHP after seeding, is shown. There is about a 1/2 inch layer of crystals in the bottom. Referring to FIG. 3C, the recorder trace of the temperature is shown. Channel 03 is a thermocouple in the sample fluid. After seeding, the sample temperature rose from -20 to -15.degree. C. due to the heat of crystallization. Time increases from right to left, with each small division representing a 2 minute time period. There is an induction period of about 6 minutes, followed by a 5.degree. C. temperature rise over 30 minutes. (Channel 04 is an extraneous flask.)
FIGS. 3A-C show 7.2 M/12.4 M BHP that was partially reacted with chlorine. Unreacted 7.2 M/12.4 M BHP at -20.degree. C. also crystallized after seeding. For comparison purposes, 7 M/10.5 M BHP was prepared in four different ways. It was been prepared at +5.degree. C. by adding the H.sub.2 O.sub.2 to the KOH, and by adding KOH to H.sub.2 O.sub.2. It was also prepared at -21.degree. C. by adding the H.sub.2 O.sub.2 to the KOH, and by adding KOH to H.sub.2 O.sub.2. All four batches yielded solid-free liquid at -20.degree. C. However, all four solutions crystallized at -20.degree. C. when nucleation seeds were introduced. Thus, 7M/10.5M BHP at -20.degree. C. is not in its equilibrium state. It reaches equilibrium by crystallizing. The unpredictable part of the process is the rate of formation of crystal nucleation seeds. Once a sufficient number of seeds form or are introduced, crystallization takes place.
Therefore, there is a need for a BHP composition that will not freeze or crystallize during the routine operation of a conventional COIL system, i.e., at temperatures down to -21.degree. C. There is also a need for a method for preventing the freezing or crystallizing of BHP during the routine operation of a conventional COIL system.